The efficiency of internal combustion engines is a direct function of the maximum thermodynamic cycle temperature of the engine. Consequently, many arrangements have been proposed for providing engine components that improve the thermal efficiency of the engine. In particular, it is well known that a thermal barrier placed near engine exhaust ports or in an exhaust manifold can significantly improve the energy efficiency of an engine.
Many attempts have been made to provide economical and effective thermally insulating members for internal combustion engines. For example, U.S. Pat. No. 1,655,149 issued Jan. 3, 1928 to C. L. Lee describes the use of a heat insulating material such as lava, porcelain, mica or asbestos as an engine exhaust duct lining. Lee proposes forming a lining as a single piece, cutting the exhaust duct in two to permit insertion of the lining, and then rejoining the duct elements together by welding. This process is costly and produces an exhaust duct having undesirable thermally induced stresses resulting from the welding operation.
To solve the problem of separating and rejoining the engine components, it has been proposed that the thermal insulating member be cast in situ with the engine member. For example, U.S. Pat. No. 3,786,795 issued Jan. 22, 1974 to Y. Kaneko et al describes an engine exhaust port construction in which a prefabricated thermally insulating member, constructed of a rigid ceramic material, is placed in a casting mold and then enveloped by molten aluminum. Similar arrangements are shown in U.S. Pat. No. 4,277,539, issued Jul. 7, 1981 to H. Keller et al, and U.S. Pat. No. 4,648,243 issued Mar. 10, 1987 to H. Korkemeir.
Because of physical strength limitations of the heretofore proposed rigid ceramic members, the metallic component of each of these constructions is formed of materials having relatively low melting temperatures, such as aluminum. Because of the relatively low temperature at which such materials are poured, these materials inherently impose lower contraction forces on an enclosed ceramic member during casting solidification than those forces imposed by higher melting temperature metallic materials, such as cast iron. Consequently, ceramic and metallic composite articles, formed by casting the metallic component about a rigid ceramic component, have heretofore been limited to the use of relatively low melting temperature materials, such as aluminum, for the metallic component of the composite article.
One solution to in situ casting of higher melting temperature metallic materials and ceramic components has been to form the ceramic component of materials that are crushable, deformable or porous. Typically, such deformable ceramic materials are enclosed between a metal sleeve and the cast component. For example, U.S. Pat. No. 2,225,807 issued Dec. 24, 1940 to C. Towler describes an asbestos thermal barrier member. Alternatively, U.S. Pat. No. 3,919,755 issued Nov. 18, 1975 to Y. Kaneko discloses a composite article in which the metallic component is cast in situ with a flexible ceramic liner.
U.S. Pat. No. 4,243,093, issued Jan. 6, 1981 to J. Nieman and assigned to the assignee of the present invention, discloses an engine manifold having inner and outer cast metallic members with an insulative portion disposed between the inner and outer members. The insulative portion of this article must necessarily be formed of a yieldable material to permit shrinkage of both the inner and outer cast members during solidification.
Insulative members constructed of porous ceramic materials must be protected by an impermeable barrier to prohibit the infiltration of hot engine gases which could cause a physical breakdown of the inner member and destruction of the bond between the member and a supporting metallic structural component. Similarly, crushable or flexible ceramic materials do not have sufficient strength properties to resist erosion during extended operation.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a thermally insulating composite article that has a rigid, high strength, inner member. It is also desirable to have an outer structural member, or component, that is formed of a material that has a high melting temperature and is cast, in situ, with the inner member.